feat: Improve OpenTelemetry trace instrumentation

[cedricziel]

This PR enhances the opentelemetry instrumentation for caddy by:

• creating client spans for reverse proxy request to fix servicegraph calculations
• injecting, conditionally, a server-timing header to propagate the traceparent down to the caller

[mholt]

Thanks @hairyhenderson !

@cedricziel Would you be able to fix up the lint errors and make sure the tests pass, then we can get this in the next beta release?

[mholt]

Thanks for the commit!

Looks like the order of the modified imports needs to be adjusted (for lint) and for tests, there's:

    module_test.go:107: Invalid traceparent: 
--- FAIL: TestTracing_ServeHTTP_Propagation_Without_Initial_Headers (0.00s)

[mohammed90]

For lint, try golangci-lint run --fix. I think that takes care of most of them.